Page 238 - Gas Adsorption Equilibria

224 Chapter 4

The adsorption chamber is assumed to have been evacuated to as low a
remnant gas pressure as possible. Upon opening the valve the gas
expands and adsorption occurs, both effects causing a decrease of the gas
pressure to a certain value After equilibrium is attained, the piston in
the storage vessel is moved to reduce the volume of the vessel and thus
increase the gas pressure till its initial value is realized again. The values
of the storage vessel’s volume and internal surface are denoted by and
respectively. The mass balance of the sorptive gas and the adsorbed
phases is

Here is the amount of mass adsorbed per unit area on the inner
surface of the storage vessel (SV). This quantity normally can be assumed to
be known from calibration measurements. Likewise denotes the mass
adsorbed per unit area on the sample material. This quantity can be calculated
from (4.75) as

Here we have assumed as in Eq. (4.75) that the storage vessel and adsorption
chamber have been manufactured from the same material having the same
surface properties so that the mass adsorbed per unit area on the inner surfaces
of the vessels is the same for both vessels. According to Eq. (4.76) the surface
adsorption of the sample material is determined by two terms
in parentheses ( ), the first of which describes the mass of gas being
transferred from the gas phase to the adsorbed phase on the surface (A) of the
sample material during the experiment. The second term in parentheses ( )
describes the change in the mass adsorbed on the internal surfaces of the
vessels, i. e. areas during the experiment. Numerical values of this
term often are small compared to those of the first term. Hence it may be
neglected, but in principle always should be considered as a correction term
which possibly should be taken into account. It should be noted that in the
balance equation (4.75) no terms related to the microbalance or its vessel
occur. This is due to the kind of “isobaric” experiment chosen, namely the
condition that the initial and the final gas pressure within the system are
the same and hence the respective terms cancel in the balance equation (4.75).